Heretofore, photographic process using silver halide has most widely been practiced, since it provides excellent photographic properties such as sensitivity, gradation, etc. as compared with, for example, electrophotographic process and diazo-type photographic process. In recent years, techniques have been developed which provide images easily in short time by employing, as photographic processing of forming images on photographic (high-sensitive) materials using silver halide, a dry processing involving heating in place of the conventional wet processing involving development in a developing solution.
Thermally developable photographic materials are known in the art, and thermally developable photographic materials and the process thereof are described in, for example, Shashin Kogaku No Kiso, pp. 553 and 555 (published by Corona Co., Ltd.), Eizo Joho, p. 40, April 1978, Nebletts Handbook of Photography and Reprography, 7th ed. (Van Nostrand Reinhold Company), pp. 32-33, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108, 1,167,777, and Research Disclosure, pp. 9-15, the June issue, 1978, (RD-17029).
Many processes have been proposed for obtaining color images in a dry manner. As to a process of forming color image by binding an oxidation product of a developing agent with a coupler, U.S. Pat. No. 3,531,286 proposes p-phenylenediamine type reducing agents and phenol or active methylene couplers, U.S. Pat. No. 3,761,270 proposes p-aminophenol type reducing agents, Belgian Pat. No. 802,519 and Research Disclosure, pp. 31 and 32 (September, 1975) propose sulfonamidophenol type reducing agents, and U.S. Pat. No. 4,021,240 proposes a combination of sulfonamidophenol type reducing agent and a 4-equivalent coupler.
However, these processes have the defect that, since an image of reduced silver and a color image are simultaneously formed in exposed portions after thermal development, the color image becomes turbid. As a means for removing this defect, the silver image is removed by a liquid treatment, or the dye alone is transferred to a sheet having another layer, for example, image-receiving sheet. However, it is not so easy to discriminate an unreacted compound from a dye and transfer only the dye.
Research Disclosure, pp. 54-58, the May issue, 1978, (RD-16966) describes a technique of introducing a nitrogen-containing hetero ring group into a dye to form a silver salt, and releasing the dye by thermal development. This technique fails to provide distinct image due to difficulty in depressing release of dye in unexposed portions, thus not being generally employed.
And, as to a process of forming positive color image according to heat-sensitive silver dye bleaching process, useful dyes and bleaching processes are described in, for example, Research Disclosure, pp. 30-32, the April issue, 1976, (RD-14433), ibid., pp. 14-15, the December issue, 1976, (RD-15227), and U.S. Pat. No. 4,235,957.
In these processes, however, an additional step and material are necessary for heating the materials with an activator sheet for accelerating a bleaching of dye superposed on them, and the resulting color images suffer gradual reductive bleaching during storage with the copresent free silver or the like.
U.S. Pat. Nos. 3,985,565 and 4,022,617 describe a process of forming color image by utilizing leuco dyes. However, with this process, stable incorporation of leuco dye in a photographic material is difficult, and the material is gradually colored during storage.